Have fun in TJ. If you do declare the meds you purchased, you are allowed 50 tabs of a controlled substance. I've been a few times and it seems like the border guards make up new rules on a regular basis. Some pharmacies sell neo percodan, which is darvon. T3s are available. If you do find oxycontin, which is unlikely, it will probably be in $1/mg or higher. Chances of getting searched at the border are low, however every once in a while it does happen.
Mexico medication prices have gone up and it's not the bargain it used to be. But you may find a good buy on one of those great black velvet paintings of a tiger.